In an ideal world, people would be just as free to move through political borders as goods or services are. Just as complete freedom of movement for goods ensures that they flow to areas where they are most demanded, so does complete freedom of movement for people ensure that they can move to where they expect to do best (one of the best ways of alleviating true poverty, incidentally). In practice, however, people come attached with things that goods do not, such as the beliefs, culture, and language of the home country, and it is these attachments and their effect on the host country that make immigration policy a tough issue.
Nonetheless, many arguments against liberal immigration policies are xenophobic bunk. Here's a Der Spiegel editorial on the subject:

'Germany is not a country of immigration,' Roland Koch said this month as he sought to revive his campaign for a third term as governor of the western state of Hesse by calling for a crackdown on 'criminal young foreigners.'

The statement, borrowed from former Chancellor Helmut Kohl, is untrue. Some 15 million people, or just under a fifth of the German population, have an immigrant background. The real message is: 'We don't want Germany to be a country of immigration.'

'Foreigners' -- they're often called that here even if they and their parents were born here -- get that message loud and clear in their everyday lives. That steely look of disapproval in shops when a customer expresses an enquiry in accented or broken German. The difficulty of finding an apartment to rent if your surname isn't Müller...

The debate over integration over the last decade has been shaped by conservative demands that immigrants adopt a German "Leitkultur," or "leading culture" -- a vague mix of Beethoven, sausages and Alpine cowbells, presumably.

I heard a German government official in Bavaria espouse the Leitkultur position last week. It's hard not to sympathize with the idea to some extent--the very reason that immigrants are coming to Germany is, after all, because it has grown into a more prosperous society with better opportunities than the immigrant's own country. Is it so unreasonable, therefore, to require that immigrants conform--even to large degree--with the proven principles of the host?

Perhaps not, but one should be careful not to conflate two issues. The first is what to do with people who want to immigrate in the future, and the second is what to do with people who've already immigrated. Germany may well not want to be a country of immigration, but that doesn't address the social problems involving the immigrants who have already entered the country. One is fundamentally about the past, the other is about the future.

As for the issue of future immigration, Germany might be reminded that the European Union, which Germany helped found sixty years ago, ostensibly guarantees four freedoms of movement: goods, services, capital, and yes, even people.